In most existing marine drives in watercraft, a marine engine is coupled to a propeller via a gear box which provides a single gear ratio. The speed of the watercraft is controlled by controlling the engine speed via a throttle. Generally speaking, watercraft are geared so that they run most efficiently at their intended cruising speed. A large yacht may be designed to cruise at 35 to 40 knots and is thus geared to be most efficient and controllable at or around that speed. However a problem with this arrangement is that such craft are very difficult to operate at the low speeds that may be required for example, when docking the craft. It is very difficult to dock a boat safely if, for example, the lowest speed that the boat will satisfactorily travel at, is around 10 knots. It is also, in some cases, desirable to have a low gear ratio for applications where increased torque is required, with a higher gear ratio for high speed operation.
Although a number of multi-speed drive transmissions have been proposed for watercraft, they suffer from a number of problems. For example, U.S. Pat. No. 6,350,165 discloses a watercraft which incorporates a two forward speed plus one reverse speed transmission. The transmission is based on a planetary gear apparatus and is consequently relatively high cost. The transmission has a further problem in that because of the way the gearing is arranged, it is not possible to vary the gear ratios easily, since all the gear wheels must be changed. It is thus harder to package and adapt planetary gearing systems to suit different applications, engine sizes and the like.
A further problematic area with watercraft, concerns “torque interrupt”. Torque interrupt occurs when a transmission system is temporarily disengaged when the gear ratio of the transmission system is changed. One common example of torque interrupt occurs when a driver changes gear in a motor vehicle. Whilst it is acceptable to have torque interrupt in a motor car, since the momentum of the vehicle will keep the vehicle moving during the period when the transmission is temporarily disengaged, in a watercraft, torque interrupt is a more serious problem because the drag on a watercraft's hull is much greater than the frictional drag on a motor vehicle and the watercraft is appreciably slowed during any period when the transmission is disengaged.
A further problem which occurs with watercraft, is “clunking” which occurs as a watercraft is put into forward or reverse gear. Whilst it is not a serious problem from an operational point of view, clearly a person spending a large sum of money to buy a watercraft, particularly at the more expensive end of the market, might expect to get a transmission system which does not “clunk” when the craft is put into gear.
Examples of the present invention seek to address or alleviate one or more of the problems of the prior art.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.